The present invention relates to an optical recording medium, and more particularly to an optical recording medium capable of forming a resin layer having an excellent surface shape and minimizing a warp while preventing an increase in a cost.
Conventionally, optical recording media represented by a CD and a DVD have been utilized widely as recording media for recording digital data. These optical recording media can be classified into read-only optical recording media which can neither once write nor rewrite data, for example, a CD-ROM and a DVD-ROM, write-once optical recording media which can once write data but cannot rewrite data, for example, a CD-R and a DVD-R, and rewritable optical recording media capable of rewriting data, for example, a CD-RW and a DVD-RW.
In order to reproduce data from these optical recording media, a laser beam set to have a power for reproduction is irradiated on the optical recording media and a reflected light is read so that the data are reproduced. A recording mark formed on the optical recording medium has a reflectance for the laser beam which is different from reflectances in the other regions. For this reason, the quantity of the reflected laser beam is varied depending on the presence of the recording mark. Accordingly, the quantity of the reflected laser beam is detected by a photodetector and is converted into an electric signal so that a reproducing signal is generated and the data are reproduced.
For this reason, it is necessary to reliably cause a laser beam reflected by the optical recording medium to be incident on the light receiving surface of the photodetector in order to read the data recorded on the optical recording medium as desired.
In the case in which a great warp is generated on the optical recording medium by a change in a temperature or a humidity during use, however, the angle of incidence of the laser beam on the optical recording medium fluctuates. For this reason, there is a possibility that the reflected laser beam might not be reliably incident on the photodetector.
In order to reproduce the data recorded on the optical recording medium as desired, accordingly, the warp of the optical recording medium is to be reduced. For example, Patent Document 1 has disclosed an optical recording medium in which the warp is reduced by the formation of a layer for preventing the warp on the back side of the optical recording medium.
The optical recording medium described in JP-A-4-195745 comprises a first dielectric layer formed on the surface side of a substrate and a second dielectric layer formed on the back side of the substrate and having a coefficient of thermal expansion which is equal to that of the first dielectric layer. In such an optical recording medium, a stress and a bending moment which are generated on the first dielectric layer are cancelled with a stress and a bending moment which are generated on the second dielectric layer depending on a change in a temperature or a humidity during use. Thus, a warp is prevented from being generated on the optical recording medium.
On the other hand, in recent years, there has been proposed a next generation optical recording medium having a larger capacity and a higher data transfer rate. In such a next generation optical recording medium, a wavelength λ of a laser beam is reduced and a numerical aperture NA of an objective lens for collecting a laser beam is increased to enhance a recording density.
When the wavelength λ of the laser beam is reduced and the numerical aperture NA of the objective lens is increased, however, there is a problem in that an angle error permitted for the tilt of the optical axis of the laser beam to the optical recording medium, that is, a tilt margin T is greatly reduced as shown in the following equation (1).
                    T        ∝                  λ                      d            ·                          NA              3                                                          (        1        )            
In the equation (1), d represents a distance from a light incidence plane to the surface of an information layer on which data are to be recorded, that is, a thickness of a layer through which a laser beam is transmitted before the laser beam reaches the information layer. As is apparent from the equation (1), the tilt margin T is decreased when the wavelength λ of the laser beam is reduced and the NA of the objective lens is increased, and is increased when the thickness d of the layer through which the laser beam is transmitted is reduced.
Therefore, the next generation optical recording medium has such a structure that a thin resin layer having a thickness of 30 to 200 μm is formed on the information layer and a laser beam is irradiated from the resin layer side to record and reproduce data. Consequently, the tilt margin is enlarged.
For this reason, the next generation optical recording medium usually has an asymmetrical structure in which an information layer and a resin layer are sequentially laminated and formed on a support substrate having a thickness of approximately 1.1 mm and two disk-shaped substrates having a thickness of 0.6 mm are stuck with the information layer interposed therebetween differently from a DVD optical recording medium having a symmetrical structure.
In the next generation optical recording medium, however, the thicknesses of the support substrate and the resin layer are different from each other. In the case in which a warp is apt to be generated on the optical recording medium, particularly, the support substrate and the resin layer are formed by different materials due to a change in a temperature or a humidity, therefore, physical properties such as a rigidity, a coefficient of linear expansion, a Young's modulus of elasticity and an internal stress of the material forming the support substrate and the material forming the resin layer are different from each other. For this reason, the warp is generated on the optical recording medium still more easily.
As described above, in the next generation optical recording medium, particularly, there is a problem in that a warp is apt to be generated. Also in the next generation optical recording medium, therefore, there has been made a trial that a resin layer having almost the same physical properties as the physical properties of a resin layer formed on the surface side of the support substrate is formed on the back side of the support substrate and stresses applied to the surface and the back face of the support substrate are offset to reduce the warp to be generated on the optical recording medium.
In order to form the resin layer having almost the same physical properties on the back side of the support substrate, however, it is necessary to carry out a spin coating step to form the resin layer, and to then invert the support substrate and to perform the spin coating step again. Furthermore, a label is generally formed on the back side of the optical recording medium in order to display data recorded on the optical recording medium or the type of the optical recording medium or to give a decorative design. A label layer for forming the label is usually obtained by a screen printing method in place of a spin coating method. For this reason, there is a problem in that a producing step becomes very complicated, causing an increase in a manufacturing cost.
In order to prevent the increase in the manufacturing cost, therefore, it is effective to form the resin layer on the back side of the support substrate by the screen printing method and to provide the resin layer and the label layer by the same forming method to simplify a manufacturing process.
However, a resin layer having a thickness of 30 to 200 μm is much thicker than the label layer, and it is necessary to use a screen printing plate which has a greater opening width and is coarser as compared with a screen printing plate to be used for forming the label layer in order to form the resin layer by the screen printing method. In the case in which the resin layer is formed by using such a coarse screen printing plate, the flatness of the surface of the resin layer is easily damaged so that the shape of the surface of the resin layer is transferred to the label layer to be formed thereon. For this reason, there is a problem in that the external shape of the label layer is deteriorated.
In the case in which the flatness of the surface of the resin layer is considerably damaged, particularly, the thickness of the resin layer becomes non-uniform. Therefore, the external appearance of the label layer is adversely affected, and furthermore, physical properties are made greatly different from those of the resin layer on the surface side of the support substrate. As a result, there is also a problem in that it is hard to prevent a warp from being generated on the optical recording medium.